Method for remelting a rod of crystallizable material by crucible-free zonemelting



Feb. 8, 1966 K. SIEBERTZ ETAL 3,234,012

METHOD FOR REMELTING A ROD'OF CRYSTALLIZABLE MATERIAL BY CRUCIBLE-FREE ZONE-MELTING 2 Sheets-Sheet 1 Filed March 7, 1960 Fi g.l

United States Patent 3,234,012 METHOD FGR REMELTING A ROD 0F CRYSTAL- LIZABLE MATERIAL BY CRUClBLE-FREE ZONE- MELTING Karl Siebertz and Heinz Henker, Munich, Germany, as-

signors to Siemens & Halske Airtiengesellschaft, Berlin, Germany, a corporation of Germany Filed Mar. 7, 1960, Ser. No. 13,309 Claims priority, application Germany, Feb. 14, 1953, S 32,193, S 32,194 7 Claims. (Cl. 75-65) This application is a continuation-in-part of the application of Siebertz and Henker, Serial No. 409,494, filed February 10, 1954, now abandoned.

This invention relates to an improvement in a cruciblefree, floating zone method for remelting crystallizable materials, particularly semiconductor materials, such as silicon and germanium.

In such methods for remelting a rod of crystallizable material, particularly semiconducting, by zonemelting, the rod is held only at its ends, preferably in vertical position, and a molten zone produced in the rod and extending over its cross section between non-melted portions of the rod, is gradually passed through the rod by displacing the energy source, producing the molten zone, along the rod axis. The molten zone is freely supported by the adjacent, non-melted material of the rod.

This crucible-free zone-melting method, which had not been published prior to the present invention, and also zone-melting with the aid of a trough-shaped crucible for supporting the rod to be remelted, serve a variety of purposes. One purpose is the purification of the ,material. This is based upon the fact that foreign substances generally have different tendencies to become dissolved in the liquid and solid phase of one and the same material. Consequently, any impurities present in the material will become enriched differentially either in the molten zone or in the solid portion of the material. Such impurities, present in the rod to be remelted, thus travel during one or more passes, always occurring in the same travelling direction, or at the ends of the rod with the high etliciency of purification, whereafter the rod ends containing the collected impurities can be cut off.

Another purpose of the zone-melting methods is the production of monocrystals. The end of a polycrystalline rod is placed into contact with the monocrystalline rod or piece of the same material arranged in a suitable crystal orientation, and the molten zone is produced first at the junction point and then gradually and successively carried through the polycrystalline rod. As a result, the material crystallizes at the rear side of the molten zone, in monocrystalline condition. Still another purpose of the zone-melting method is the doping of semiconductor rods. The doping substances are introduced into the molten zone and are then built into the recrystallizing semiconductor material at the clear side of the molten zone.

Gne important advantage of the more recent, and not previously published, crucible-free mode of the zone-melting method, as compared with the older known method employing a crucible, is that of affording a higher degree of purity and an undisturbed monocrystal formation of the product being processed. This advantage is due to the fact that the'danger of impurities diffusing into the molten zone from vessel walls are avoided, so that the disturbances in monocrystal formation caused by such undesired impurities are also eliminated. On the other hand, however, the crucible-free zone-melting is technologically more difficult to carry out, since the freely floating, liquid material may easily drop off.

The invention will be described below with reference to the drawing, which illustrates an apparatus employed --duced in thickness in its upper portion.

3,234,612 Patented Feb. 8, 1966 lCC to carry out the formation of the molten zone, its relative movement lentghwise of the semiconductor rod, the displacement of one end thereof with respect to the other,

and the rotation of the rod.

FIG. 1 is a vertical section of one form of apparatus;

FIG. 2 is a vertical section of a modification;

FIG. 3 is a detail view of the molten zone.

Investigations carried out with the crucible-free zonemelting method, which resulted in the present invention, have shown it to be disadvantageous to keep the supported ends of the rod being r-emelted at an invariable distance from each other during the zone-melting process. This is particularly so with respect to the remelting of rods consisting of semiconducting material such as silicon or germanium. These substances have the unusual property of expending appreciably when solidifying, this expansion being up to 10% with silicon. Consequently, the molten zone occupies a smaller volume thanthe solid material that was previously melted for producing the molten zone. Thus, a considerable jump in volume takes .place upon transition from solid to liquid condition. This causes a considerable non-uniformity of the material crystallizing out ofthe molten zone.

Due to these phenomena, the molten zone must, of necessity, become constricted in its middle so that a waist is formed, because the liquid material .at the boundary of the molten zone is held fast at the adjacent solid material by adhesive forces. The effect of gravity is also to be taken into account. If, for obtaining uniform conditions at the recrystallization faces, the zone-melting is performed with a vertically mounted rod, the molten zone will bulge outwardly at its lower portion due to its own weight, whereas the zone becomes correspondingly re- Consequently, the waist due to the weight of the molten zone and also due to the volumetric jump discussed above, is not symmetrically located with respect to the middle cross-sectional plane of the molten zone, but is somewhat displaced upwardly. Hence, the molten zone, under the efiect ofgravity is not completely symmetrical.

Before continuing with the discussion of the basic phenomena involved in the invention, we shall describe a specific embodiment in conjunction With the drawing. Referring to FIG. 1, the rod 1 to be processed is mounted at both ends in respective holders 2, '3 and is surrounded by a ring-shaped heater 4 for melting an axially narrow cross-sectional zone 5 as described above with reference toFIG. 1. Holder 3 is mounted on a support 21 which is slidably connected with a base plate 22 and can be displaced in the axial direction of rod 1 by means of a screw spindle 23. A bell-shaped housing 24 sealed against the base plate 22 encloses the rod 1 to permit operating in vacuum or in a protective atmosphere. The holder 2 is mounted on a shaft25 that passes through the housing to the outside and carries a gear 26 to be driven from a motor 27. The heater ring 4 is mounted on an axially displaceable rod 28 that extends to the outside of the housing 24 where it is connected by a rope 29 with a reel 30.

Operating the reel, for instance by-means of another motor, causes the rod 28 and the heater 4 to progress along the rod 1, for instance, in the direction of the arrow 6 for zone-melting the rod in the manner described. During the processing, the spacing between the holders 2 and 3 can be varied for controlling the cross section of the processed portion of material. Also during the zonemelting, the'motor 27 may be operated to keep the upper portion of rod 1 in revolution. If desired, another motor may be provided for also revolving the holder 3 with the lower portion of rod 1, as shown in FIG. 2.

A magnet coil 31 is mounted on rod 28 in' concentric relation to heater ring 4 to produce "during the melting operation a magnetic field to counteract the tendency of the molten material to run off. For the same purpose the device is also shown equipped-with nozzles 32 (FIG. 1) that, when in use, blow a stream of gas upwardly against the molten zone. The gas may be identical with the one ing, drawing of crystals, or dispersing of additive substance maybe utilized individually or in any desired combination with each other. Hence, depending upon the effect to be utilized, one or the other of the above-mentioned advantages will deserve preference, so that the.

selection of the various arrangements and modifications described in the foregoing depends upon the requirements or desiderata of each particualr application. For instance, it is understood thatthe wall of housing 24 can be provided with a sighting opening covered by glass,or that a sighting instrument can .be inserted thereinto. Also, the upper and lower portions of rod 1 can be rotated in unison in the same direction by a motor geared to both ends, as shown in FIG. 2.

Referring to FIG. 2, it will be seen that this figure provides modified drive means. The rod 28 is reciprocated by a gear. The motor 27 turns upper and lower gears 26,26e in unison, through gears 26d, 26c, rod 33,-and upper gears 26a, 26b.

The invention is based upon'the recognition that with respect to the cross section of the material crystallizing the boundary of the recrystallization face, i.e. at the rear side of the travelling zone, has a critical influence. this shape is kept invariable, the cross section of the solidified material also remains invariable. Otherwise, however, an increase .or reduction of this cross section takes place. The shape of the molten zone at the recrystallization face, however, has generally a tendency to vary con-.-

tinuously. Only if the limit angle a of the molten zone at the recrystallization face is adjusted to a givencritical out of the molten zone, the shape of the molten zone at a and can maintain this'value without disturbance, will i the shape of the molten zone at the recrystallization face. and hence the cross section of the recrystallizing material remain unchanged. The term limit angle denotes the angle formed by the generatrix lines of the solid, already recrystallized rod portion, with the generatrix lines of the molten zone at the contact location between the molten zone and the solid rod portion. The angle isso counted that for at smaller than 180 C. an outward bulging of the molten zone at the location of contact is defined.

The critical value ar of the limit angle is a constant of.

the particular material being zone-melted. This value does not depend upon the dimensions of themolten zone.

The critical value is approximately 170 with silicon andcorresponds to a slight outward bulging of the molten done with the aid of the method according to the present invention.

The invention, relating to a method for the remeltin'g I of rods of crystallizable, particularly semiconducting, material by the crucible-free zone-melting principle requires that the cross section of the material crystallizing out of '1 the molten zone be controlled, particularly increased or reduced, by mutual axial displacementiof the twosolid rod portions between which the molten zone is carried.

The limit angle, or actually adjusting itself is determined the rod ends, in the stabilizing direction or sense, the.

and to some extent also by the temperature of the molten zone. The larger the volume,;and hence the weight of the molten zone, the larger will become thev outward bulging at the lower end of the melting zone for a constant size of the recrystallization face, and the more deeply willfthe waist cut into the upperrange of the molten zone. Thisdete rmines the limit angle at the; upper end, as well as. the limit angle at the lower end ofv the molten zone, because the upper limit angle is the more narrow the less deep the waistconstricts the molten zone; and the lower limit angle'is more narrow if theweight of the molten zone is smaller. Furthermore, the; limit angle at the upper end of the molten zonehas a somewhat different value from thelimit angle at the lower end. This can be explained by the above-mentioned asymmetry in the shape of the molten zone caused by the efiect of gravity upon the molten zone. In' contrast tothe critical limit angle 1 therefore, the limit angles thatactually form themselves are not constants of the materialbut they also depend essentially upon the size of the molten 'zone.

For the zone-melting according to the. present invention, the limit angle at the. rear side of the molten zone, i.e. at the recrystallization face, is critical.

By virtue of the method accordingtothe invention, performed while maintaining the adjustment of the heat source for producing the molten zone at a constant value, the volume and hence the weight of the molten zone can be controlled or regulated at will. This is because, due to the pulling apart of the solid rod portions and hence of the molten zone, the melted marginal portions of the molten zone pass out of the, ,direct action range of the energy source and solidify, i.e.:freeze. This takes material away. from the molten zone so that its volume and weight are reduced. Conversely, whenrthesolid rod or stub portions are caused to approach each other, more material is melted at their respective :ends; so that the volume of the molten zone is increased in this manner.

The change in' the weight of the molten zone produced by these mutual displacements manifests itself in a change of the eife'ctive limit angle a at the upper and at the lower end of the molten zone. As a result, the molten zone can be adjusted. and kept at. any desired value, particularly at the critical value a within the limits of mechanical stability of the molten zone.

The zonermelting is performedina processing vessel that permits continous observation of the molten zone. Consequently, any change-in the marginal angles and in the shape of the molten zone occurring'during the travel of the molten zone can be observed. The pulling-apart or pushing-together; of the solid rod portions is preferably efiectedwith the aid of mechanisms which engage the holders of the rod ends. and which displace these ends withoutvibration and in a definedmanner. Suitable as such mechanisms, for example, are micrometer guiding device for screw spindles. They' can be so designed with out difiiculty that the. rod ends-whichi carry the molten zone can be adjusted to a.defined.mutual spacing, such 1 excited, to oscillations.- Hencethe-occurrence, of oscillations is indicative of the fact. that the: molten'zone has commenced to become instable. By timely displacing of danger of the molten zone dripping off can still be avoided,

Genera1ly, the attendant .will readilyrigain the requisite. V

skill to perform the pulling-apart or pushing-together of the molten zone with suificient care, and will rapidly recognize the limits of operation.

In a device for performing the method according to the invention it is not necessary to have'both rod ends mounted in displaceable holders. The pulling-apart or pushing-together can be performed in the same manner if only one of the two holders is displaceable.

The operation of the method depends upon whether the rod is to be thicker or thinner than the original rod after remelting, and whether the rod cross section is to remain constant or is to vary continuously over the length of the rod. Taken as an example is a rod of silicon or germanium, constituting a material which occupies the larger volume when liquid than in solid condition. It is preferable to pass the molten zone from below upwardly through the length of the rod because the lower limit angle of the molten Zone can be more easily adjusted than the upper angle. The further description therefore specifically applies to this mode of operation.

(a) When the cross section of the recrystallized material is to remain constant and is to be equal to the diameter of the original rod, the limit angle at the recrystallization face is to be adjusted accordingly, to the critical value a and this value is to be kept constant during travel of the molten zone.

Due to the described jump in volume occurring with materials like silicon or germanium the molten zone, immediately upon liquefication, oonstricts itself so strongly that bothlimit angles are smaller than ca Therefore, in order to bring the limit angle up to the critical value, the solid rod portions between which the molten zone is held must be pushed toward each other. This adjustment of the limit angle to the critical value a is preferably effected with a stationary molten zone. When thereafter the molten zone, while maintaining this critical value for the limit angle, is successively passed lengthwise through the rod to be remelted, the'material crystallizes out of the molten zone with a constant cross section corresponding to that of the original rod.

Whether during this operation the limit angle maintains the adjusted value (1 without using additional eX- pedients or whether it shows a tendency toward spontaneous change, depends upon the fact whether or not the speed of the recrystallization at the rear side of the molten zone (i.e. in the present case at the lower end of the zone) is equal to the speed of the melting occurring at the forward (upper) end. If the two speeds are not equal, the volume of the moltenzone' and thus the size of the limit angle will change. Such irregularities must be compensated by employing the method according. to the invention. To accomplish this the following is to be taken into account:

Each enlargement in volume of the molten zone results in an increase in the outward bulging at the lower side of the molten zone, whereas a reduction in volume causes a flattening of the bulge. Consequently, a volume increase of the molten zone causes a diminution of the lower limit angle; and avolume decrease causes an increase of that angle.

Pulling the molten zone apart, in accordance with the method of the invention, thus causes an increase of the limit angle or (reduction of the bulge), whereas a pushingtogether of the molten zone causes a decrease of the limit angle at (increased outward bulging).

Consequently, during the travel of the molten zone, the pulling-apart or pushing-together of the solid rod ends compensates for any departure of the limit angle at the recrystallization face (at the lower end of the molten zone) from the adjusted critical value ar such as may be due to a change in volume of the molten zone. As a result, a rod having a uniform cross section over its length can be produced from a rod of variable cross section.

However, if the cross section of the rod to be remelted is constant, and the limit angle prior to commencing the travel of the zone is adjusted to the critical value a then a subsequent regulation of the spacing between the two solid rod portions is not necessary, because then the material immediately crystallizes out ofthe molten zone with a constant cross section corresponding to that of the rod being remelted, and the speed of the melting corresponds to that of the recrystallization.

The material of the molten zone expands when solidifying. It is therefore preferable to slightly pull the solid rod ends away from each other when the molten zone reaches the other rod end, so that'the cross section of the molten zone after solidification becomes equal to the cross section of the material previously crystallized out of the molten zone.

(b) We shall now describe the operation employed when the cross section of the recrystallized material is to be smaller or larger than the cross section of the original rod while remaining uniform over the length of the remelted rod. Depending upon whether the material crystallized out of the molten zone is' to have a larger or smaller cross section than the original rod, the limit angle a, after producing the molten zone, must be adjusted either to a greater or smaller value'than corresponds to the critical value 01 As a result, when the moltenzone travels a small distance forward, a continuous increase or decrease in cross section of the recrystallizing material is obtained, and the cross section of the recrystallization face is brought to the desired size.

This expedient is unnecessary when the size of the recrystallization'face already has the desired value due to the application of a seed crystal, and the molten'zone is first produced at the junction point between seed crystal and the rod to be remelted.

When recrystallization face has the desired size, the limit angle at the recrystallization face, during further travel of the molten zone, must be adjusted to the critical value m in order to keep the cross section of the recrystallizing material constant. However, since the recrystallizing rod has a cross section different from that of the rod portion being melted, the melting speed no longer is equal to the recrystallizing speed. The molten zone therefore exhibits the tendency to continuously vary its volume, so that the limit angle will continuously vary if the method according to the invention is not being employed.

When the recrystallizing material and hence the recrystallization face possesses a cross section smaller than that of the rod portion being melted, the volume of the molten zone must thus become continuously larger. This corresponds to an increase in the outward bulge of the molten zone at the recrystallization face, and thus also to a continuous decrease in value of the limit angle a. For this reason, the two rod portions that carry the molten zone between them must be pulled apart continuously so that the limit angle remains at'the value required for obtaining a constant recrystallizing cross section. The speed of the pulling-apart motion (v) is generally slight and can be determined by the formula:

wherein V denotes the traveling speed of'the molten zone, Q the cross section of the melted semiconductor, and q the cross section of the recrystallized rod portion.

Conversely, when the cross section of the original rod is to be smaller than the cross section of the recrystallized rod portion, the two solid rod portions must be continuously pushed together in order to keep the volume of the molten zone constant and the limit angle at the value ca If the cross sections of the rod portion being melted and the cross section intended to be recrystallized differ a great amount, it is advisable to stepwise adapted the cross section of the recrystalling material to the intended value by passing the molten zone several times along the rod.

(c) The cross section of the material crystallizing out of the molten zone is to vary continuously along the length of the rod. In this case, the limit angle must be kept at a value which is either greater or smaller than the critical Value 41 For obtaining a uniform rate of cross-sectional change, the limit angle is likewise to be kept at a constant value.

As explained, the invention offers procedures for con-' trolling at will the cross section of the recrystallizing ma terial resulting from a crucible-free zone-melting process. The method can be carried out under protective gas or in vacuum. The use of vacuum has the advantage that.

than corresponds to their mutual spacing during the melt 1 ing. Due to the effect of centrifugal force, the limit angle a at the recrystallization face can be reduced. This can be used to advantage particularly in cases where the cross section of the recrystallized rod is to be larger than the cross section of the original rod.

' We claim:

1. A floating zone, crucible-free method for remelting a rod of crystallizable semiconducting material, comprising supporting the rod only at its end portions, in a substantially vertical position, heating a zone of said rod to produce a molten zone extending over the cross section of the rod, the molten zone being bordered on opposite sides by solidmaterial of the rod, gradually displacing the molten zone along the longitudinal axis of the rod by gradually relatively displacing the rod and a source of heating energy that produces the molten zone, the molten 2. A floating zone, crucible-free method for remelting a rod of crystallizable semiconducting material, comprising supporting the rod at its end portions, heating a zone of said rod to produce a molten zone extending over the cross section of the rod, the molten zone being bordered 1 on opposite sidesby solid material of the rod, gradually displacing the molten zone along the longitudinal axis of the rod by gradually relatively displacing the rod and a source of heating energy that produces the molten zone, the molten zone being supported by adherence to bordering solid material, and mutually displacing the two rod ends that carry the molten zone axially with respect to each other so as to control the cross section of the material crystallizing out of the molten zone, the semiconductor material being taken from the group consisting of silicon and germanium, which are characterized by expansion upon solidifying.

3. A floating zone, crucible-free method for remelting a rod of crystallizable semiconducting material, the semiconductor material being one which expands upon soliditying, comprising supporting the rod only at its ends, in a substantially vertical position, heating. a zone of said rod to-produce a molten zone extending over the cross section of the rod, the molten zone. being bordered on op- I posite sides by solid material of the rod, gradually displacing the molten zone upwardly along the longitudinal axis of the rodby'gradually relatively displacing the rod and a source of heating energy that produces the molten zone, the molten zone being limited in mass to.that supportable by adherence to the bordering solid material, and mutually displacing the two rod ends that carry the molten zone axially with respect to'each other so asito maintain constant the cross section of the material crystallizing out 1 of the molten zone, so as to obtain a recrystallized rod of determined outline;

4. A floating zone, crucible-free method for remelting a rod of crystallizable semiconducting material, the material being one which expands upon solidifying, comprising pporting the rod only at its ends, in a substantially vertical position, heating a zone :of said. rod to produce a molten zone extending over the cross section of the rod, the. molten zonebeing bordered on oppositesides.

by solid material. of the .rod, gradually displacing the molten zone upwardly along thev longitudinal-axis of the rod by gradually relatively displacing the rod and a source of heating energy that produces the moltenzone, the molten zone being limited in mass to that supportable by adherence to bordering solid material, the molten material recrystalliziug at the underside of the zone and mutually displacing the two rod ends, that carry the molten zone axially toward each other so as to enlarge the cross section of the material crystallizing out of the molten zone.

5. A floating zone, crucible-free method for remelting a rod of crystallizable semiconducting material, comprising supporting the rod only at its ends, in .a substantially vertical position, heating a zone of said' rod to produce a molten zone extending over the-cross section ofthe rod, the molten zone being bordered on opposite sides by solid material of the rod, gradually displacing the molten zone along the longitudinal. axisofthe rod by gradually relatively displacing the: rod and a source of heating energy that produces themolten zone,.the' molten zone being supported by adherence to bordering. solid material, and mutually displacing the .two rod ends that carry the molten zone axially towardeach other, so to obtain an enlargement of the diameter: of. the material. crystallizing from the -molten zone,-a11d augmenting said enlargement by;

subjecting the molten zone to centrifugal force.

6. The methoddefined in claim 5, all of the recited operations taking place simultaneously.

7. The method defined in claim 5, the material being silicon, all of the operations taking place simultaneously.

References Cited by theExaminer;

OTHER REFERENCES Journal of Applied Physics, vol. 24, No. 12, December 1953, article by Keck et al., pages 1479-1481. 1

DAVID. L. RECK, Primary Examiner.-

RAY K. WINDHAM, WINSTON A. DOUGLAS, .MARCUS U. LYONS,BENJAMIN HENKIN,

Y Examiners. 

1. A FLOATING ZONE, CRUCIBLE-FREE METHOD FOR REMELTING A ROD OF CRYSTALLIZABLE SEMICONDUCTING MATERIAL, COMPRISING SUPPORTING THE ROD ONLY AT ITS END PORTIONS, IN A SUBSTANTIALLY VERTICAL POSITION, HEATING A ZONE OF SAID ROD TO PRODUCE A MOLTEN ZONE EXTENDING OVER THE CROSS SECTION OF THE ROD, THE MOLTEN ZONE BEING BORDERED ON OPPOSITE SIDES BY SOLID MATERIAL OF THE ROD, GRADUALLY DISPLACING THE MOLTEN ZONE ALONG THE LONGITUDINAL AXIS OF THE ROD BY GRADUALLY RELATIVELY DISPLACING THE ROD AND SOURCE OF HEATING ENERGY THAT PRODUCES THE MOLTEN ZONE, THE MOLTEN ZONE ADHERING TO THE BORDERING SOLID MATERIAL, AND MUTUALLY DISPLACING THE TWO ROD ENDS THAT CARRY THE MOLTEN ZONE AXIALLY WITH RESPECT TO EACH OTHER SO AS TO CONTROL THE CROSS SECTION OF THE MATERIAL CRYSTALLIZING OUT OF THE MOLTEN ZONE. 